As it is well known, a probe card is essentially a device adapted to electrically connect a plurality of contact pads of a microstructure, in particular an electronic device integrated on wafer, with corresponding channels of a test apparatus that performs the functionality verification, in particular electrical, or generally tests it.
The test performed on integrated devices in particular serves to detect and isolate faulty devices already at the manufacturing stage. Usually, the probe cards are thus used for electrically testing devices integrated on wafer before cutting or singling out the same and before the assembly thereof within a package for containing chips.
A probe card comprises a probe head in turn essentially including a plurality of mobile contacting elements or contact probes provided with at least one contact portion or tip for a corresponding plurality of contact pads of a device to be tested. The terms end or tip are hereinafter used to indicate an end portion, not necessarily sharply pointed.
Thus, it is known that the effectiveness and reliability of a measurement test depends, among other factors, also on the obtainment of a good electric connection between the device and the test apparatus, thus, on an ideal electrical probe/pad contact.
The types of probe heads which are widely used in the technical field considered herein for testing integrated circuits include the so-called probe heads having cantilevered probes, also referred to as cantilever probe heads, which have probes protruding like a fishing rod over a device to be tested.
In particular, a cantilever probe head of the known type usually supports a plurality of flexible, generally wire-like probes, with preset electrical and mechanical properties. The probes, cantilevered projecting from the cantilever probe head, are substantially shaped to form a hook, due to the presence of a substantially elbow bended terminal portion having a generally obtuse internal angle.
The good connection between the probes of a cantilever probe head and the contact pads of a device to be tested is due to the pressure of the probe head on the device itself, the probes being subjected thereby to a vertical flexion (with respect to the plane defined by the device to be tested) in the opposite direction with respect to the movement of the device towards the probe head.
The hooked shape of the probe is such that, during the contact with the contact pads of the device to be tested and the displacement of the probes upwards beyond a pre-established contact point, usually referred to as “overtravel”, the points of contact of the probe slide on the contact pads over a length commonly referred to as “scrub”.
Also known in the field are the probe heads having vertical probes and referred to by the term “vertical probe heads”. A vertical probe head essentially comprises a plurality of contact probes held by at least one pair of plates or guides substantially plate-like and parallel to each other. Such guides are provided with special holes and they are arranged at a given distance with respect to each other so as to leave a free zone or air gap for the movement and possibly deformation of the contact probes. In particular the pair of guides comprises an upper guide or die and a lower guide or die, both provided with guide holes within which the contact probes axially slide, usually made of special alloy wires with good electrical and mechanical properties.
The good connection between the contact probes and the contact pads of the device being tested is also in this case due to the pressure of the probe head on the device itself, the contact probes, movable within the guide holes made in the upper and lower guides, being subjected to a flexion during such pressure contact within the air area between the two guides, and to a sliding within such hole guides.
Furthermore, the flexion of the contact probes in the air gap may be assisted through a suitable configuration of the probes or of the guide thereof, in particular using pre-deformed contact probes or suitably shifting the plates that comprise them.
FIG. 1A schematically illustrates a probe head 1 comprising at least one upper plate or guide 2 and a lower plate or guide 3, having respective upper guide holes 2A and lower guide holes 3A within which there slides at least one contact probe 4.
The contact probe 4 has at least one end or contact tip 4A. In particular the contact tip 4A abuts on a contact pad 5A of a device 5 to be tested, performing the mechanical and electrical contact between said device and a test apparatus (not represented) regarding which the probe head forms an end element.
Generally, there are used probe heads with probes not fixedly constrained, but kept interfaced with a board, in turn connected to the test apparatus: this is the case of non-locked contact probes.
In this case, as illustrated in FIG. 1A, the contact probe 4 has a further contact tip 4B, in practice indicated as contact head, towards a plurality of contact pads 6A of the board 6. The good electrical contact between probes and the board is analogously due to the contact with the device to be tested by pressing the probes on the contact pads of the board.
In addition, the board 6 is held in position by means of a stiffener 8. The assembly of the probe head, the board and the stiffener forms a probe card, indicated in its entirety with 10 in FIG. 1B.
Thus, in the vertical probe technology it is also important to guarantee the good connection of the contact probes with the testing apparatus, in particular in correspondence of the contact heads thereof and thus of the board.
Another critical parameter in the production of a probe head is the distance (the so-called pitch) between the centres of the contact pads on the device to be tested. The pitch of the integrated electronic devices has become smaller and smaller as the corresponding manufacturing technology has been progressing, leading to a high packing of the contact probes 4 in the probe head 1, with the ensuing problems regarding positioning to avoid the mutual contact thereof. Such distance restrictions are slightly less strict instead as regards the contact pads on the board 6, given that such pads are suitable to be spaced further apart and arranged more freely, in particular in a more orderly manner, compared to those of the device to be tested.
There are known various technologies for obtaining the probe card 10 of the testing apparatus.
In particular, a first solution exploits the techniques of the printed circuits for obtaining the card 10, which is also commonly referred to as PCB. This technology allows obtaining cards with active areas even of large dimensions, but with major restrictions with respect to a minimum value that can be attained for the pitch in conditions of high density of the contact pads on the device to be tested.
Also known is the ceramic based technology or MLC (Multilayer Ceramic). This technology allows attaining extremely small pitches and much higher densities compared to the PCB technology, but it entails limitations regarding the maximum number of signals that can be used for the test and on the maximum dimension of the active area that can be accommodated on the card.
Lastly, there can be used the so-called hybrid technology in which the probe card is interfaced with an intermediate plate, commonly indicated as interposer, in turn connected to a mechanical support, commonly indicated as a plug and connected by means of welding bridges to the card. This technology offers considerable flexibility in terms of surface, pitch and signal density, but it is limited as regards the maximum number of signals that can be treated, also revealing poorer electromagnetic performance. Last but not least, the difficult automation capacity thereof also represents a disadvantage of the hybrid technology.
The mutual positioning of the elements forming the card is also a key parameter. In particular, various test methods need that the probe card be capable of withstanding high temperatures. However, in such case the thermal expansions of such elements that form the probe card may hinder the correct working thereof. Actually, the elements that form the probe cards of the known type are usually fastened by means of screws. Thus, during testing under high temperatures, due to the various coefficients of thermal expansion of the materials such elements are made of and the fastening obtained by the screws, the elements tend to arc, causing the malfunctioning of the probe card as a whole, even leading to lack of contact with the contact pads of the device to be tested.